Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net Sales	$ 473,823	$ 487,814	$ 495,101	$ 468,269	$ 420,636	$ 445,030	$ 454,603	$ 445,897	$ 1,925,007	[1]	$ 1,766,166	[1]	$ 1,776,167	[1]
Gross Profit	80,599	75,602	89,966	92,098	68,720	83,395	92,931	93,499	338,265		338,545		308,241
Operating income	30,831	30,309	38,961	46,059	9,932	28,738	42,916	44,607	146,160		126,193		122,790
Interest, net	(2,826)	(2,763)	(2,863)	(2,992)	(3,350)	(2,824)	(3,417)	(3,614)	(11,444)		(13,205)		(14,533)
Income Before Income Taxes	28,532	29,312	37,063	44,330	7,009	27,143	39,196	40,468	139,237		113,816		102,856
Net Income	9,886	21,853	27,896	31,912	8,411	21,367	28,501	27,919	91,547		86,198		76,037
Net income attributable to Stepan Company	$ 9,884	$ 21,899	$ 27,882	$ 31,913	$ 8,417	$ 21,362	$ 28,496	$ 27,916	$ 91,578		$ 86,191		$ 75,968
Diluted earnings per share	$ 0.42	$ 0.94	$ 1.19	$ 1.37	$ 0.36	$ 0.92	$ 1.24	$ 1.22	$ 3.92		$ 3.73		$ 3.32

[1] Net sales are attributed to countries based on the location of the Company facility making the sales.